The present invention relates to a new and improved method of enciphering and deciphering audio information which is subdivided into partial blocks along a time axis, the partial blocks being mutually interchanged according to key information, and wherein the incoming analog audio signals are subdivided into a number of frequency bands each of which is assigned an information channel. This invention also relates to apparatus for the performance of the aforesaid method which incorporates at least one input side-branching filter for subdividing the incoming analog audio signals into a number of frequency bands each determining a respective information channel.
The heretofore known methods and apparatuses for enciphering speech sounds are essentially subdivided into two groups:
The first group contemplates converting the analog speech signals into digital signals, for instance by means of a so-called vocoder (voice coder), a pulse-code-modulation system (PCM-modulation system) or a delta-modulation system. The pulses are linked or coupled in conventional manner with one another by means of key pulses which are generated by a key generator. The thus enciphered characters are transmitted to the receiver end or side of the system and at that location converted, in appropriate manner, again into deciphered analog speech signals.
This group of prior art equipment affords the advantage of a high quality of the tone or sound and a high redundancy of the transmitted information. Moreover, there are so many possible variations during enciphering, that the security against decryption is extremely high.
The foregoing prior art systems have several drawbacks; a large bandwidth is required for transmission purposes and the equipment is sensitive to phase shifts in the transmission system.
According to a second group of prior art equipment the analog speech signals are not transformed into digital signals. The speech information is subdivided into partial groups along the frequency axis and/or time axis. These partial groups are then permutated by a key information generated by a key generator, so that there is produced a new sequence of the partial groups. Yet, the information as such is still accommodated within the same frequency band and is of the same nature as the original speech information. As a result, there can be employed for the transmission of the information, without disadvantage, transmission systems for speech transmission possessing a corresponding limited bandwidth.
Consequently, there is realized the advantage that extremely large bandwidths are not required for transmission, and phase shifts in the transmission system have practically no influence upon the quality of the transmitted information.
Yet, the second group of equipment is associated with the drawbacks that the variation possibilities for permutation of the partial groups is relatively limited, so that there is hardly possible realization of any effective security against improper access to the plain text information by unauthorized third persons.